In drilling operations, a drilling rig is typically used to drill a wellbore to recover oil or gas reserves disposed below the Earth's surface. For safety and other reasons, the driller maintains control of the well by controlling the pressure of the drilling fluid, sometimes referred to as mud, within the wellbore. The driller may control the pressure of the drilling fluid by adjusting one or more of the flow rate of mud that the mud pumps deliver downhole, the rotation rate of the top drive/rotary table that rotate the drill string, and the position and speed of the block during tripping, drilling, stripping, and other well construction operations, as well as through the introduction of weighting agents. The drilling fluid is typically pumped through the interior passage of the drill string, the drill bit, and back to the surface through the annulus between the wellbore and the drill pipe. On the surface, the returning fluids may be processed through a mud-gas separator, a shale shaker, or other fluids system before being recirculated for further use downhole.
To maintain well control, the driller typically maintains the pressure within a safe pressure window bounded by the pore pressure and the fracture pressure. The pore pressure typically refers to the pressure of the fluid (liquid or gas) inside the pores of the rock. If the pressure in the annulus falls below the pore pressure, formation fluids may flow into the wellbore and well control may be lost. The fracture pressure typically refers to the pressure at which the formation hydraulically fractures or cracks and may vary as a function of the depth of the well. If the pressure in the annulus rises above the fracture pressure, wellbore fluids may enter the formation and well control may be lost.
During the drilling of subsea wells, the hydrostatic pressure of the drilling fluid is typically maintained at a pressure higher than the pore pressure as a primary barrier to the influx of formation fluids into the wellbore. A blow-out preventer (“BOP”) is typically placed over the wellbore on the subsea surface as a secondary barrier. If during drilling, a zone is encountered where the pore pressure is higher than the fluid pressure inside the wellbore, an influx of formation fluids may be introduced into the wellbore and the marine riser. The formation fluids may include liquids, gases, or combinations thereof. Such an occurrence is commonly referred to as a kick and may occur not only during drilling, but also during completion, work-overs, or interventions.
When a kick is taken, the unknown fluids, which may include some mixture of drilling fluids and/or formation fluids, may decrease the density of the fluid in the wellbore annulus, such that an increasing amount of formation fluids enter the wellbore. In such circumstances, control of the well may be lost due to the breach of the primary barrier. Typically, during drilling operations, the BOP remains open and the return of fluids from the well are directed through a fluid return line to a fluids system on the surface. If the amount of gas is small, the fluids returned under normal drilling operations are directed to the shale shaker. If the amount of gas is higher than an acceptable amount, the fluid return is directed to the mud-gas separator to remove the entrained gases from the fluids. When a kick is unexpectedly taken, as soon as the kick is detected, the BOP is closed, and the fluid return is directed to the mud-gas separator to perform the well control operation and return the well to a safe condition so that drilling can be resumed. Because of the delay in detecting the kick and closing off the BOP, formation fluids may enter the marine riser. The presence of formation fluids containing gas in the marine riser poses substantial risk to the safety of the rig, the crew, and the environment as the riser is typically open to the atmosphere without the possibility of closing it off.
Some rigs are equipped with a device to controllably seal the top of the marine riser. In these rigs, there is typically a fluid return line connecting the riser to the well control choke manifold, which directs fluids to the mud-gas separator. Managed pressure drilling (“MPD”) rigs also close the top of the mariner riser, but typically have a separate and dedicated MPD choke manifold. The fluid return line from the marine riser can be routed to the well control manifold or to the MPD choke manifold. And from these manifolds there are fluid lines directing fluid flow into the mud-gas separator. The main purpose of these fluid lines is to route the fluid return believed to be contaminated with gas to the proper equipment on the rig, namely, the mud-gas separator, to safely remove the gas. However, the process of eliminating gas from the wellbore or riser is performed manually by a user controlling the choke manifold as well as other equipment on the rig. The entire operation is conducted at very low flow rates, to avoid overflowing the mud-gas separator and to simplify the manual control of the pressures during the operation. The manual process is inefficient, prone to error and failure, and presents a substantial safety and environmental risk.